The overall objective of this proposal is to gain additional information about the molecular organization and function of chromosomes in higher animal cells. In particular, chromomeric DNA sequences will be characterized and compared from two functionally related loci which are located on polytene chromosomes in Chironomus tentans. In salivary glands, these chromomeres form giant puffs, known as Balbiani rings (BRs), which are sites actively engaged in tissue-specific gene expression. Translational units within BR transcripts will be identified by in vitro protein synthesis and mapped by electron microscopy. Subsequently, physical maps will be constructed indicating the relative cleavage sites of restriction endonucleases in and around BR transcription units. By combining the information on these maps with recombinant DNA techniques, a series of highly purified probes will be constructed which collectively represent BR transcription units, flanking regions, and remaining chromomeric DNA sequences. These probes will then be used to search for gene- and/or chromomere-associated homologous sequences in other chromosomal loci via nucleic acid hybridization in situ. The studies will permit a comparison of gene structure to be made in inactive versus active tissues, enhance the identification of signals possibly involved in coordinate gene expression at related loci, and begin to examine the nature of "excess" chromomeric DNA.